The present invention generally relates to communications interference detection and mitigation systems and specifically to a narrow beam antenna diversity system and method for an RF data transmission system.
Currently, there are several so-called xe2x80x9clast milexe2x80x9d and xe2x80x9clast footxe2x80x9d data transmission systems which are designed to deliver high speed and/or high data capacity from the internet backbone to the end user. Several such systems use RF transmissions to replace copper wire or fiber optic cables. Some of these systems are called point to point or point to multipoint systems and operate in various licensed and unlicensed RF bands. A fundamental characteristic of most existing systems is that their RF transmissions occur in a frequency spectrum protected and regulated by a government body. These protected frequency spectrums, or bands, are licensed to certain license holders and only a few may operate in any given physical area depending upon the number of licenses available.
There are only a limited number of licensed bands in any geographic area, thus, in order to widen the choices consumers have, it is desirable for service providers to be able to use unlicensed RF bands to provide a capability to deliver high speed, high capacity data services. In addition, a service provider may not hold licenses in every geographic area that it desires to provide service. Therefore, use of unlicensed frequency bands may allow a service provider coverage or a greater geographical area.
In 1997 the FCC created a wireless arena called Unlicensed National Information Infrastructure (U-NII). System operators are free to operate wireless equipment in three sub-bands (5.15 to 5.25 GHz, 5.25 to 5.35 GHz and 5.725 to 5.825 GHz) without acquiring a licensed frequency spectrum. The FCC specifies the conditions and rules for operating wireless equipment in the U-NII frequency band. However, operators are not protected from possible interference from other U-NII operators transmitting in the vicinity or even other type of systems which utilize the same frequencies.
The IEEE, a standards group, is defining a wireless LAN standard, referred to as IEEE 802.11a for operation in the U-NII band. Equipment that conforms to this standard will operate indoors at the lower and middle frequency sub-band i.e. 5.15 to 5.25 GHz and 5.25 to 5.35 GHz. The ETSI BRAN group in Europe has defined an air interface standard for high-speed wireless LAN equipment that may operate in the U-NII frequency band. Equipment that is compatible with this standard may cause interference with use of the U-NII bands.
Prior art wireless communication systems that operate in non-line-of-sight environments benefit from multiple receive paths. If the signal paths are not correlated and the signal level on one path drops due to destructive combining of multiple signals, the signal level on the other path may not be experiencing the same fading. There are several prior art methods to reduce correlation between signal paths, the most common is spatial separation of antennas. Usually the antennas need to be separated by at least 10 wavelengths in order to be effective, which is impractical in many applications, such as cellular phone handsets. However, in cellular system base stations, antennas can be spatially separated to provide spatial diversity to mitigate the effects of fading. Other diversity techniques include angular diversity, polarization diversity or a combination of both. When angular and polarization diversity are employed, a spatial separation requirement for diversity may be eliminated.
In prior art fixed wireless data transmission systems, diversity inputs are generally not employed for subscriber stations. Diversity inputs may only be employed at the base station or hub, if used at all. Problematically, fixed wireless systems generally use directional antennas, which requires the antenna to be relatively large in order to obtain a directional beam. In order to avoid doubling the width of the antenna, the two diversity paths can be vertically separated.
In a fixed point to multipoint wireless data transmission system problematic variations arise in the path between a hub and a subscriber resulting in signal fading and multipath effects. For example, the movement of trees and traffic or buildings swaying result in significant signal fading and multipath effects. Additionally, data traffic in a network, such as a LAN, WAN, Intranet or the Internet, tends to be asymmetrical. Generally, more data is transmitted to a subscriber than from a subscriber to the network backbone. Therefore, in a wireless RF data transmission environment, it is desirable to provide a stable broadband link from the data transmission hub of such a system to a subscriber. In the reverse direction from a subscriber station to a hub, it is desirable to provide a stable link as well. Hence, it is desirable to provide a subscriber system which can receive employing antenna diversity, and to direct a subscriber to transmit to a hub using a most advantageous single antenna beam since the base station may also employ receive diversity.
The present system and method provides narrow antenna beam polarity, angular and/or spatial diversity for subscriber stations in a point to multipoint RF data transmission system. At the subscriber station the system comprises a multibeam antenna generating a plurality of antenna beams. Coangular pairs of the antenna beams comprise two beams having orthogonal polarity. Both horizontal and vertical polarizations, or two other orthogonal polarizations, may be utilized at the subscriber antenna. Each of the pairs are angularly diverse from other pairs. The subscriber transceiver has a plurality of receiver inputs and at least one transmitter output, generally, two inputs and one output. At the hub the bit error rate of transmissions from a subscriber and/or signal levels of available frequencies and polarizations are monitored and the optimal beam, frequency and polarization for transmissions from the subscriber are determined. Any changes in frequency, beam or polarization is communicated to the subscriber unit to initiate a change.
Polarization diversity for reception of transmissions from a single hub is preferably implemented by the subscriber unit employing one of the coangular pairs of antenna beams for reception. The hub transmits to the subscriber unit employing a corresponding pair of antenna beams with frequencies and polarizations matching the coangular pair of subscriber antenna receiving beams. Alternatively, as described below with reference to the embodiment of FIG. 2, angular diversity may be provided to a subscriber by transmitting to the subscriber from two separate hubs using angularly diverse antenna beams which correspond to angularly diverse receive beams used by the subscriber station. These beams may or may not have polarization diversity as well.
When a single transmitting antenna is used at a base station the transmission environment may change the characteristics of the transmitted signal. Obstructions, conditions and variations in the transmission path between a single base station and a subscriber station may result in signals arriving from multiple angles at the subscriber station. Therefore, the subscriber station may advantageously employ angular diversity. Similar environmental conditions may cause signals to arrive at a subscriber station with a variation in polarization which may be best received using a polarization orthogonal to the polarization of the original transmission.
One of the advantages of a fixed wireless system is a larger antenna footprint than normally exist in the mobile environment. This allows use of multiple antennas and antenna beams at a subscriber station, as well as the hub. Using horizontally spatially separated, narrow beam antennas is impractical at a subscriber station because of the associated physical antenna width. However, it is advantageous to use a narrow antenna beam with either a different polarization, than the primary subscriber antenna beam, or to use a different azimuth than the primary antenna beam. Such polarization diversity and angular diversity is practical with a small form factor antenna. This will provide two or more inputs to the subscriber receiver and hence diversity. Alternatively, a second antenna, physically spaced a small vertical distance from a first antenna, but covering the same space between the hub and the subscriber station can also provide diversity.
At the subscriber station of one embodiment of the system there is a multibeam antenna generating a plurality of antenna beams. Coangular pairs of the antenna beams comprise two beams having orthogonal polarity. Each of the pairs are angularly diverse from other pairs. By way of example, if there are 120 degrees of coverage by a subscriber antenna, this coverage can be provided by 12 radiation patterns. A single radiation beam pattern is, in this example, 20 degrees wide with a horizontal polarization. An additional 20 degree, coangular beam pattern with a vertical polarization is available. Therefore, in this example, there would be six antenna beams in azimuth with two different polarizations, thereby providing 12 radiation patterns. The subscriber unit has a plurality of receiver inputs and at least one transmitter output to utilize the available beams. In this manner, diverse paths into the subscriber receiver unit are provided. Alternatively, vertically spatially separated antennas may be utilized to provide 24 radiation patterns.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.